There are various applications of face recognition. In general, a face recognition system compares a new image (the “candidate image”) with an existing image of a known person's face (the “reference image”), and attempts to determine whether the candidate image is of the same person as the reference image. For a given database of reference images, it may be possible to take a new candidate image and determine which, if any, of the people in the database is shown in the new image.
One application of face recognition is to identify people in photos that are uploaded to social networking sites, photo-sharing sites, or similar types of services. This situation differs from the many other face recognition scenarios, since a social context exists that can help to infer the identity of people in the photos. In the general case of the face recognition problem, it might be assumed that the only thing that is known to help identify a face is how visually similar two face images are to each other, according to some comparison metric. However, in a social networking site, there tends to be a relatively small set of people of whom a given user tends to upload images. For example, a user might tend to upload images of his or her friends, or might tend to upload images of some subset of his or her friends (who can be identified from previously-uploaded photos). However, in many systems that allow users to be tagged in photos, this information is not used.